Igor Bdikin

Crystal structure and multiferroic properties of Gd-substituted BiFeO3

Room-temperature crystal structure, local ferroelectric, and magnetic properties of the Bi1−xGdxFeO3 (x=0.1,0.2,0.3) polycrystalline samples have been investigated by x-ray diffraction, piezoresponse force microscopy, and magnetometry techniques. Performed measurements have revealed a sequence of the composition-driven structural phase transitions R3c→Pn21a (occurs at x∼0.1) and Pn21a→Pnma (takes place within the concentrational range of 0.2<x<0.3). The latter structural transformation is attributed to the substitution-induced suppression of the polar displacements. Gd substitution has been shown to effectively induce the appearance of the spontaneous magnetization, thus indicating a promising way for improving multiferroic properties of antiferromagnetic BiFeO3.

Large-area high-throughput synthesis of monolayer graphene sheet by Hot Filament Thermal Chemical Vapor Deposition

We report hot filament thermal CVD (HFTCVD) as a new hybrid of hot filament and thermal CVD and demonstrate its feasibility by producing high quality large area strictly monolayer graphene films on Cu substrates. Gradient in gas composition and flow rate that arises due to smart placement of the substrate inside the Ta filament wound alumina tube accompanied by radical formation on Ta due to precracking coupled with substrate mediated physicochemical processes like diffusion, polymerization etc., led to graphene growth. We further confirmed our mechanistic hypothesis by depositing graphene on Ni and SiO2/Si substrates. HFTCVD can be further extended to dope graphene with various heteroatoms (H, N, and B, etc.,), combine with functional materials (diamond, carbon nanotubes etc.,) and can be extended to all other materials (Si, SiO2, SiC etc.,) and processes (initiator polymerization, TFT processing) possible by HFCVD and thermal CVD.

Nanoscale domains and local piezoelectric hysteresis in Pb(Zn1/3Nb2/3)O3-4.5%PbTIO3 single crystals

High-resolution domain studies have been performed in Pb(Zn1/3Nb2/3)O3-4.5%PbTiO3 (PZN-PT) single crystals via piezoresponse force microscopy (PFM). Irregular domain patterns with the typical sizes 20–100 nm have been observed on the (001)-oriented surfaces of unpoled samples. On the contrary, (111) crystal cuts exhibit normal micron-sized regular domains with the domain boundaries directed along allowed crystallographic planes. The existence of nanodomains in (001)-oriented crystals is tentatively attributed to the relaxor nature of PZN-PT where small polar clusters may form under zero-field-cooling conditions. These nanodomains are considered as the nuclei of the opposite polarization state and can be responsible for the smaller coercive field for this particular crystal cut. However, local piezoelectric hysteresis performed by PFM on the nanometer scale indicates similar switching behavior of PZN-PT for both investigated crystallographic orientations.

Anomalous polarization inversion in ferroelectrics via scanning force microscopy

Local poling of ferroelectrics by the sharp conducting tip of a scanning force microscope (SFM) is studied experimentally and theoretically. The formation of the inverse domain under the SFM tip, where the polarization is oriented in the direction opposite to that of the poling field, is reported for bulk ferroelectrics (single crystals of solid solutions PbZn1/3Nb2/3O3–PbTiO3). This finding confirms earlier results on ferroelectric thick films, thus proving the universality of the anomalous polarization inversion in ferroelectric media. It is shown that the inverse domain grows with the increase of the poling voltage and duration and remains stable for a long time after the removal of electric field. The growth process is described by a dynamic model assuming that the appearance of inverse domains is due to a local internal electric field directed against the poling one. This field is attributed to the space charge formed beneath the SFM tip due to injection of charge carriers and their subsequent drift and trapping. Poling voltage and poling time dependences of the domain size are correctly described by the model. Implications of the anomalous polarization inversion for the domain engineering and dense data storage in ferroelectrics are discussed.

Temperature-driven phase transformation in self-assembled diphenylalanine peptide nanotubes

Diphenylalanine (FF) peptide nanotubes (PNTs) represent a unique class of self-assembled functional biomaterials owing to a wide range of useful properties including nanostructural variability, mechanical rigidity and chemical stability. In addition, strong piezoelectric activity has recently been observed paving the way to their use as nanoscale sensors and actuators. In this work, we fabricated both horizontal and vertical FF PNTs and examined their optical second harmonic generation and local piezoresponse as a function of temperature. The measurements show a gradual decrease in polarization with increasing temperature accompanied by an irreversible phase transition into another crystalline phase at about 140‐150 ◦ C. The results are corroborated by the molecular dynamic simulations predicting an order‐disorder phase transition into a centrosymmetric (possibly, orthorhombic) phase with antiparallel polarization orientation in neighbouring FF rings. Partial piezoresponse hysteresis indicates incomplete polarization switching due to the high coercive field in FF PNTs. S Online supplementary data available from stacks.iop.org/JPhysD/43/462001/mmedia (Some figures in this article are in colour only in the electronic version)

Structural, morphological and piezoresponse studies of Pr and Sc co-substituted BiFeO3 ceramics

Pristine (BiFeO3), Pr and Sc co-substituted Bi0.9Pr0.1Fe1−xScxO3 (0.01 ≤ x ≤ 0.07) ceramics have been investigated for their structural (including x-ray diffraction and Raman), morphological and piezoresponse behaviour. Secondary phases observed in pristine BiFeO3 (BFO) ceramic were significantly suppressed in the co-substituted samples. Grain size decreased in the co-substituted samples. Raman study revealed ten active phonon modes which remained almost invariant with co-substitution. Out-of-plane piezoresponse (OPP-PFM) exhibited negative self-polarization effect in the virgin state (without any poling) which has been explained in terms of built-in electric field. The self-polarization effect is largest in 3% Sc co-substituted sample. Poling with ±30 V dc voltage demonstrated both positive and negative domains. Maximum difference in the peak values (from histograms) of such opposite domains was observed in the Bi0.9Pr0.1Fe0.99Sc0.01O3 sample. Information from local measurements (such as PFM) should be useful in deciding such multiferroic materials for device application.

Room temperature surface piezoelectricity in SrTiO3 ceramics via piezoresponse force microscopy

SrTiO3 ceramics are investigated by piezoresponse force microscopy. Piezoelectric contrast is observed on polished surfaces in both vertical and lateral regimes and depends on the grain orientation varying in both sign (polarization direction) and amplitude. The observed contrast is attested to the surface piezoelectricity due to the flexoelectric effect (strain gradient-induced polarization) caused by the surface relaxation. The estimated flexoelectric coefficient is approximately one order of magnitude smaller as compared to that recently measured in SrTiO3 single crystals. The observed enhancement of piezoresponse signal at the grain boundaries is explained by the dipole moments associated with inhomogeneous distribution of oxygen vacancies.

Breakdown into nanoscale of graphene oxide: Confined hot spot atomic reduction and fragmentation

Nano-graphene oxide (nano-GO) is a new class of carbon based materials being proposed for biomedical applications due to its small size, intrinsic optical properties, large specific surface area, and easy to functionalize. To fully exploit nano-GO properties, a reproducible method for its production is of utmost importance. Herein we report, the study of the sequential fracture of GO sheets onto nano-GO with controllable lateral width, by a simple, and reproducible method based on a mechanism that we describe as a confined hot spot atomic fragmentation/reduction of GO promoted by ultrasonication. The chemical and structural changes on GO structure during the breakage were monitored by XPS, FTIR, Raman and HRTEM. We found that GO sheets starts breaking from the defects region and in a second phase through the disruption of carbon bonds while still maintaining crystalline carbon domains. The breaking of GO is accompanied by its own reduction, essentially by the elimination of carboxylic and carbonyl functional groups. Photoluminescence and photothermal studies using this nano-GO are also presented highlighting the potential of this nanomaterial as a unique imaging/therapy platform.

Molecular modeling of the piezoelectric effect in the ferroelectric polymer poly(vinylidene fluoride) (PVDF)

In this work, computational molecular modeling and exploration was applied to study the nature of the negative piezoelectric effect in the ferroelectric polymer polyvinylidene fluoride (PVDF), and the results confirmed by actual nanoscale measurements. First principle calculations were employed, using various quantum-chemical methods (QM), including semi-empirical (PM3) and various density functional theory (DFT) approaches, and in addition combined with molecular mechanics (MM) methods in complex joint approaches (QM/MM). Both PVDF molecular chains and a unit cell of crystalline β-phase PVDF were modeled. This computational molecular exploration clearly shows that the nature of the so-called negative piezo-electric effect in the ferroelectric PVDF polymer has a self-consistent quantum nature, and is related to the redistribution of the electron molecular orbitals (wave functions), leading to the shifting of atomic nuclei and reorganization of all total charges to the new, energetically optimal positions, under an applied electrical field. Molecular modeling and first principles calculations show that the piezoelectric coefficient d33 has a negative sign, and its average values lies in the range of d33 ~ −16.6 to −19.2 pC/N (or pm/V) (for dielectric permittivity ε = 5) and in the range of d33 ~ −33.5 to −38.5 pC/N (or pm/V) (for ε = 10), corresponding to known data, and allowing us to explain the reasons for the negative sign of the piezo-response. We found that when a field is applied perpendicular to the PVDF chain length, as polarization increases the chain also stretches, increasing its length and reducing its height. For computed value of ε ~ 5 we obtained a value of d31 ~ +15.5 pC/N with a positive sign. This computational study is corroborated by measured nanoscale data obtained by atomic force and piezo-response force microscopy (AFM/PFM). This study could be useful as a basis for further insights into other organic and molecular ferroelectrics.

Evidence of ferroelectricity and phase transition in pressed diphenylalanine peptide nanotubes

Self-assembled peptide nanotubes (PNT) are unique nanoscale objects having a great potential for a multitude of applications. Strong piezoactivity and polar properties in aromatic dipeptides were recently observed in stand-alone nanotubes using piezoresponse force microscopy and 2nd harmonic generation. In this work, we report macroscopic dielectric and polarization vs. field measurements on pressed PNTs before and after annealing at 150 °C. The results corroborate nanoscale study and present a clear evidence of ferroelectric-like behaviour and phase transition in this technologically important material. The dielectric constant of PNT pellets obeys apparent Curie-Weiss (CW) law with the CW constant C ≈ 230 °C and transition temperature at T ≈ 142 °C.